Lysosomal storage disorders are a group of inherited disorders characterized by deficiencies in specific enzymes in the body, which results in the body's inability to break down metabolic substances. As an example, Fabry disease is a lysosomal storage disorder seen in one out of every 40,000 people. It is caused by a deficiency in the enzyme alpha-galactosidase which results in the body's inability to break down specific fatty substances called globotriaosylceramides. A second example is Gaucher disease, a lysosomal storage disorder caused by an inability to break down fatty substances or lipids called glucosylceramides (also called glucocerebrosides). Individuals with Gaucher disease do not make glucocerebrosidase, an enzyme needed to break down these fatty substances. These fatty substances then accumulate in cells of the liver, spleen, and bone marrow. A third example is Pompe disease, a lysosomal storage disorder caused by a deficiency in the enzyme acid alpha-glucosidase, which is needed to break down certain sugars called glycogen. When the enzyme acid alpha-glucosidase is missing, glycogen accumulates in various tissues and organs in the body.
Lysosomal storage disorders are, for the most part, childhood disorders although some manifest in adulthood. In most of them, patients are normal at birth and have progressive neurological deterioration beginning at some later time. The clinical phenotype depends on the type and severity of the biochemical defect. Some of these lysosomal disorders, such as Pompe disease and Krabbe disease, manifest primarily in infancy. There have been ongoing efforts in developing methods to detect such disorders before the onset of clinical symptoms so that therapeutic interventions can be initiated.
Over the past decade laboratories that test for metabolic disorders have introduced tandem mass spectrometry into their newborn screening programs. Tandem mass spectrometry continues to gain popularity in the clinic because this technology allows for assay of many metabolites in a single sample. For example, this technology has been implemented as a routine clinical practice for the detection of hereditary metabolic disorders in newborns using dry blood spot samples (Schulze A et al., Pediatrics 2003; 111:1399-406). Although lysosomal enzyme activities can be quantified using tandem mass spectrometry (Gelb M H et al., Clinical Chemistry 50:10, 1785-1796, 2004), published assay methods have not been readily adaptable to a clinical setting due to cumbersome procedures and harsh assay components such as chloroform.
A second commonly used clinical assay protocol is the enzyme linked immunosorbent assay (ELISA). Several biomarkers are presently used to detect and monitor Gaucher disease (see, for example Aerts, J M, and Hollack, C D, Bailliere's Clin. Haematol, 1997; 10:691-709; Deegan P B, et al., Blood Cells Mol Dis, 2005; 35:259-67; Beutler, E, et al., J Exp Med, 1976; 143:975-80). An ELISA for detection of antibodies to aglucerase in which the active component is modified glucocerebrosidase has also been reported (Richards S M, et al., Blood, 1993; 82:1402; Rosenberg, M. et al, Blood, 1999; 93: 2081-2088). However, these assays do not directly detect lysosomal enzyme activity, and instead detect levels of indirect markers of disease.
Thus, there is a continuing need for improving the methods and compositions for detecting lysosomal disorders.